Opto-electronic oscillators (“OEOs”) are electro-optic systems that may be used to generate microwave frequency signals having high stability and low phase noise. Typically, OEOs include a single-mode laser and an electro-optic modulator coupled to at least one active feedback loop having an open-loop gain greater than one to sustain microwave frequency oscillation. The active feedback loop includes a photodetector that converts the optical signals output from the electro-optic modulator into microwave frequency electrical signals that are, in turn, used to control the modulation of light passing through the electro-optic modulator. Thus, the active feedback loop converts the laser's output photon energy into microwave signals.
Many oscillation modes can oscillate simultaneously in the OEO as long as the gain of the active feedback loop exceeds the loop's losses. The active feedback loop typically includes a radio frequency (“RF”) amplifier that amplifies the electrical signal output from the photodetector. The OEO's active feedback loop also includes an RF bandpass filter that is used to select a single mode of oscillation for the OEO's. The RF bandpass filter is also used for coarse frequency tuning of the OEO's single oscillation mode. The electrical signal output from the RF bandpass filter is the OEO's output microwave signal. The electrical signal output from the RF bandpass filter is also used to control modulation of the light that propagates through the electro-optic modulator.
One challenge associated with OEOs is that the OEO's RF bandpass filter can only provide for coarse frequency tuning. Also, the RF bandpass filter does little to reduce phase noise in the output microwave signal. Furthermore, the RF bandpass filter does not provide for fast tunability of the OEO's oscillation frequency. Therefore, there is a need for an OEO that provides for fine frequency tuning with reduced phase noise and fast tunability.